FIG. 1 is a perspective view illustrating the main portion of a light receiving element 100 as an example of light receiving elements. FIG. 2 is a cross-sectional view of the light receiving element 100 taken along a broken line II-II in FIG. 1.
The light receiving element 100 illustrated as FIGS. 1 and 2 includes a photo-detector unit 101 provided over a substrate 114, and a waveguide unit 111 provided over the same substrate 114. The waveguide unit 111 includes a core 112 having a rib-like shape. A signal light propagates within a projection 113 of the core 112, and enters the photo-detector unit 101.
The photo-detector unit 101 has a structure in which the core 112, an n-type semiconductor layer 102, an i-type absorbing layer 103, a p-type upper clad layer 104, and a p-type contact layer 105 are laminated from the substrate 114 side. The photo-detector unit 101 has a mesa structure including the p-type contact layer 105, the p-type upper clad layer 104, and the i-type absorbing layer 103. The core 112 exists both in the photo-detector unit 101 and the waveguide unit 111.
As illustrated as FIG. 2, in the light receiving element 100, the signal light propagates below the projection 113 of the core 112 in the waveguide unit 111, and enters the core 112 in the photo-detector unit 101. Part of the incident signal light seeps out into the n-type semiconductor layer 102. As the signal light further propagates in the photo-detector unit 101, the signal light spreads to the i-type absorbing layer 103, and is absorbed in the i-type absorbing layer 103.
The n-type semiconductor layer 102, the i-type absorbing layer 103, and the p-type upper clad layer 104 form a PIN-type photo diode (hereinafter referred to as PD). A p-side electrode and an n-side electrode (not illustrated) are connected to the p-type contact layer 105 and the n-type semiconductor layer 102, respectively. By applying a predetermined voltage between the p-side electrode and the n-side electrode, with the p-side electrode at a negative potential and the n-side electrode at a positive potential, photocarriers (holes and electrons) generated by light absorption in the i-type absorbing layer 103 are detected via the p-type upper clad layer 104 and the n-type semiconductor layer 102. Thus, the photo-detector unit 101 detects the signal light as an electrical signal (photocarrier current), and outputs a detection signal (photocarrier current) corresponding to the intensity of the signal light.
FIG. 3 illustrates the main portion of a light receiving element 300 as another example of light receiving element. FIG. 4 is a cross-sectional view of the light receiving element 300 taken along a broken line IV-IV in FIG. 3.
The light receiving element 300 illustrated as FIGS. 3 and 4 has a structure different from that of the light receiving element 100 illustrated as FIGS. 1 and 2. The light receiving element 300 includes a photo-detector unit 301 provided over a substrate 314, and a waveguide unit 311 provided over the same substrate 314.
The waveguide unit 311 has a structure in which an n-type lower clad layer 302, a core 312, and an upper clad layer 313 are laminated from the substrate 314 side. The waveguide unit 311 has a mesa structure including the upper clad layer 313 and the core 312. A signal light propagates in the core 312, and enters the photo-detector unit 301.
The photo-detector unit 301 has a structure in which the n-type lower clad layer 302, an i-type absorbing layer 303, a p-type upper clad layer 304, and a p-type contact layer 305 are laminated from the substrate 314 side. The photo-detector unit 301 has a mesa structure including the p-type contact layer 305, the p-type upper clad layer 304, and the i-type absorbing layer 303.
The core 312 and the i-type absorbing layer 303 are both formed on the n-type lower clad layer 302 that is shared by the photo-detector unit 301 and the waveguide unit 311. The core 312 is connected to a side face of the i-type absorbing layer 303.
As illustrated as FIG. 4, in the light receiving element 300, the signal light propagates in the core 312 in the waveguide unit 311, and directly enters the i-type absorbing layer 303 in the photo-detector unit 301. The incident signal light is absorbed in a region near the end of the i-type absorbing layer 303 from which the signal light enters.
The n-type lower clad layer 302, the i-type absorbing layer 303, and the p-type upper clad layer 304 form a PIN-type photodiode. A p-side electrode and an n-side electrode are connected to the p-type contact layer 305 and the n-type lower clad layer 302, respectively. By applying a predetermined voltage between the p-side electrode and the n-side electrode, with the p-side electrode at a negative potential and the n-side electrode at a positive potential, photocarriers (holes and electrons) generated by light absorption in the i-type absorbing layer 303 are detected via the p-type upper clad layer 304 and the n-type lower clad layer 302. Thus, the photo-detector unit 301 detects the signal light as an electrical signal (photocarrier current), and outputs a detection signal (photocarrier current) corresponding to the intensity of the signal light.
An example of the two light receiving elements 100 and 300 illustrated as FIGS. 1 to 4 is discussed in Japanese Laid-open Patent Publication No. 2003-163363 and Andreas Beling et al. J. Lightwave Tech., VOL. 27, NO. 3, pp 343-355, Feb. 1, 2009.
FIG. 5 illustrates an example of simulated density distributions of photocarriers generated in the photo-detector units 101 and 301. The vertical axis represents the density of photocarriers as normalized on the basis of a predetermined value. The horizontal axis represents location inside the PD in each of the photo-detector units 101 and 301. In this specification, the term “location inside the PD” refers to a location inside the PD along the direction in which the corresponding core extends, that is, the direction of travel of the signal light, and means a location inside the PD included in the photo-detector unit with reference to the end from which the signal light enters. Also, the term “PD length” refers to the length of the PD in the photo-detector unit along the direction in which the corresponding core extends, that is, the direction of travel of the signal light, and means the length of the PD in the photo-detector unit with reference to the end from which the signal light enters.
In FIG. 5, the curve indicated by (a) represents photocarrier density distribution in the photo-detector unit 101 illustrated as FIGS. 1 and 2, and the curve indicated by (b) represents photocarrier density distribution in the photo-detector unit 301 illustrated as FIGS. 3 and 4.
In the photo-detector unit 101 of the light receiving element 100, the signal light seeps out into the i-type absorbing layer 103 after propagating through the core 112 and the n-type semiconductor layer 102 in the photo-detector unit 101 by a predetermined distance, and absorption thus takes place. Accordingly, as is apparent from the distribution curve (a) in FIG. 5, the peak of the photocarrier density distribution occurs at a location separated by a predetermined distance from the end of the photo-detector unit 101 from which the signal light enters. Moreover, the overall density distribution also spreads to a location farther away from the end of the photo-detector unit 101, so the distribution has a large spread as a whole.
Therefore, in the photo-detector unit 101, the PD length of the photo-detector unit 101 is set to a sufficient length for obtaining sufficient absorption efficiency. However, making the PD length of the photo-detector unit 101 longer increases the size of the capacitor including the n-type semiconductor layer 102, the i-type absorbing layer 103, and the p-type upper clad layer 104, causing an increase in capacitance of the photo-detector unit 101. Thus, the cut-off frequency derived from the CR time constant becomes lower in the transmission path between the light receiving element 100 and the subsequent electrical circuit. Consequently, in the subsequent electrical circuit that receives an electrical signal outputted from the light receiving element 100, the level of the input signal attenuates at high frequencies, making it difficult to appropriately process the input signal also at high frequencies.
As a result, with the structure of the light receiving element 100 illustrated as FIGS. 1 and 2, it is difficult to supply a detection signal with sufficient signal level to the subsequent electrical circuit while ensuring high light absorption efficiency.
On the other hand, in the photo-detector unit 301 of the light receiving element 300, the signal light having propagated through the core 312 directly enters the i-type absorbing layer 303. Consequently, large absorption takes place in the vicinity of the end of the photo-detector unit 301. Thus, as is apparent from the distribution curve (b) in FIG. 5, photocarriers generate and their density becomes high within a narrow range near the end of the photo-detector unit 301 from which the signal light enters.
Therefore, in the photo-detector unit 301, high light absorption efficiency is obtained even if the PD length is short. However, since the rising edge of the photocarrier density distribution is so large near the end of the photo-detector unit 301 that in the case of high intensity light input where the intensity of the inputted signal light is high, the density of photocarriers locally generated near the end of the photo-detector unit 301 becomes excessively high. As a result, in the photo-detector unit 301, a large electric field is created between the p-type upper clad layer 304 and the n-type lower clad layer 302 by the excessive locally generated photocarriers, in a direction opposite to the electric field created by the above-mentioned voltage applied between the p-side electrode and the n-side electrode. The electric field created by the excessive locally generated photocarriers acts to cancel out the electrical field created by the above-mentioned voltage applied between the p-side electrode and the n-side electrode. This makes it difficult for the photo-detector unit 301 to appropriately detect the photocarriers (holes and electrons) generated by light absorption in the i-type absorbing layer 303 via the p-type upper clad layer 304 and the n-type lower clad layer 302. Consequently, the high frequency property for high intensity signal light deteriorates.
As a result, with the structure of the light receiving element 300 illustrated as FIGS. 3 and 4, it is difficult to perform an output operation adapted to high intensity light input.